Solar cells are devices that convert light energy into electrical energy. These devices are also often called photovoltaic (PV) cells. Solar cells are manufactured from a wide variety of semiconductors. One common semiconductor material is crystalline silicon.
Solar cells have three main elements: (1) a semiconductor; (2) a semiconductor junction; and (3) conductive contacts. Semiconductors such as silicon may be doped n-type or p-type. If an n-type silicon and p-type silicon are formed in contact with one another, the region in the solar cell where they meet is a semiconductor junction. The semiconductor absorbs light. The energy from the light may be transferred to the valence electron of an atom in a silicon layer, which allows the valence electron to escape its bound state leaving behind a hole. These photogenerated electrons and holes are separated by the electric field associated with the p-n junction. The conductive contacts allow current to flow from the solar cell to an external circuit.
FIG. 1 shows the basic elements of a prior art solar cell. The solar cells can be fabricated on a silicon wafer. The solar cell 5 comprises a p-type silicon base 10, an n-type silicon emitter 20, bottom conductive contact 40, and a top conductive contact 50. The p-type silicon base 10 and the n-type silicon emitter 20 contact one together to form the junction. The n-type silicon 20 is coupled to the top conductive contact 50. The p-type silicon 10 is coupled to the bottom conductive contact 40. The top conductive contact 50 and the bottom conductive contact 40 are coupled to a load 75 to provide it with electricity.
The top conductive contact 50 (“front contact”), comprising silver, enables electric current to flow into the solar cell 5. The top conductive contact 50, however, does not cover the entire face of the cell 5 because silver is not entirely transparent to light. Thus, the top conductive contact 50 has a grid pattern to allow light to enter into the solar cell 5. Electrons flow from the top conductive contact 50, and through the load 75, before uniting with holes via the bottom conductive contact 40.
The bottom conductive contact 40 (“rear contact” or “back contact”) usually comprises aluminum-silicon eutectic. This conductive contact 40 typically covers the entire bottom of the p-type silicon 10 in order to maximize conduction. The aluminum is alloyed with silicon at high temperatures of approximately 750 degrees Celsius, well above the aluminum-silicon eutectic temperature of 577 degrees Celsius. This alloying reaction creates a heavily-doped p-type region at the bottom of the base and gives rise to a strong electric field there. This field aids in repelling the light-generated electrons from recombining with holes at the back contact so that they can be collected more efficiently at the p-n junction.
The interface between silicon and a conductive contact is typically an area having high recombination. For example, the back surface recombination velocity of an aluminum back surface field across the entire back surface may be 500 centimeters per second or more. High back surface recombination velocities decrease cell efficiency.